As a tire rotationally engages a roadway its individual tread design elements cause disturbances in surrounding air molecules, generating sounds in the audio frequency spectrum. The geometry of the individual tread design elements, commonly known as pitches or pitch elements, has been found to control those specific frequencies at which sounds are generated and the relative energy content (i.e., amplitude) of each such frequency. Where pitches of constant dimensions have been used to completely surround a pneumatic tire, the majority of the energy occurs at a single resonant frequency and harmonics of that frequency. This tonal concentration is highly disturbing to the occupants of the vehicle on which the tire is mounted.
Efforts along two basic lines have been taken to ameliorate the objectionable noise generated by rotating pneumatic tires. Some have attempted to modulate deleterious noise producing frequencies by sequencing pitch elements randomly or in accordance with preselected algebraic or trigonometric relations. For example, in the publication "Quieting Noise Mathematically--Its Application to Snow Tires" (S.A.E. Paper No. 690520) a mathematical method is disclosed for modifying the location of particular tread elements to more uniformly distribute audio noise throughout the frequency spectrum. Others have varied the pitch lengths on individual pitch elements again in a random or preselected algebraic or trigonometric manner. Most recently work has been done in simultaneously utilizing both techniques, as exemplified by Great Britain patent application No. 2,014,520A and U.S. Pat. No. 3,926,238.
The principal deficiency of the prior art has been the necessity for design by trial and error. Heretofore a tread pattern had to be built and tested before its exact noise characteristics were known. Indeed, in a very recent U.S. Pat. No. 4,178,199, the method used to reduce the amount of noise generated by a particular tread pattern required comparison to a tread pattern whose audio excitation frequencies were previously ascertained. In short, there does not appear to exist any device or method for obtaining the tread pattern required to generate a preselected audio output spectrum.